It’s complicated, so let me try to explain. Throw comes from the Germanic Old Saxon word thrawan. It has not changed very much. Catch comes from the Old French cachier, which came from the Latin captare. That seems plain enough so far, but where did caught with that GH come from? Well, catch (the noun) was thought to be a synonym of latch, which has Germanic roots, and the past tense of catch gradually acquired the Germanic GH. It was a slow process of change, and until the 19th century the past tense of catch was usually catched.
Over the centuries, many English words have changed or fallen out of fashion. We no longer pronounce the -ED ending by either pronouncing the D or as a second syllable as Shakespeare did. Sleeped, for example, has been simplified to slept. Many verbs have three separate words for present, past, and perfect tenses. Present tense I RING the bell. Simple past tense: Yesterday I RANG the bell, Perfect tense: For many days I HAVE RUNG that bell. But for many other verbs, the simple past tense and the perfect tense are the same word: Present tense: I THINK about verbs. Simple past tense: I THOUGHT about verbs yesterday. Perfect sense: I HAVE THOUGHT about verbs for many weeks.
Nowadays, most people want uniformity, especially in language, and when they hear
ring rang rung
sing sang sung
swim swam swum
sink sank sunk
they see nothing wrong with
fling flang flung
bring brang brung
think thank thunk
wring wrang wrung.
These extremely irregular verbs may sound funny, but they’re also incorrect, and if you use them you may be accused of being illiterate. If I write brung, my editor will be most annoyed. But not only does it fit the pattern, it also gets rid of that ungainly word brought with its useless GH. It is hardly surprising that brung is often heard being used in casual conversation. If I used the verb thunk in conversation or in my writing, I’m sure that would also raise eyebrows. But it too fits the pattern and it also gets rid of an ungainly word, thought with its archaic GH.
To claim that people who use brung and thunk are uneducated is not quite correct. We don’t have an academy in the English-speaking world to ordain what is correct or incorrect. We don’t have an Academie Française that dictates how we speak and write. English is a democratic language. It doesn’t need a dictator. Also, English is always changing, and the flang, brang, thank, and wrang are logical. Perhaps one day two or three of those words will be acceptable. But probably not to my editor. [Editor’s note: John is correct.]